Fix Py_UNREACHABLE test on Python 3.9
Fix Py_UNREACHABLE test on Python 3.9

By default, the macro no longer fails; it just instructs the
compiler to treat the state as unreachable.
It can be made to fail using Py_DEBUG or RANDALL_WAS_HERE.

encukou/py3c,encukou/py3c,encukou/py3c